The present invention relates to a radiation treatment planning system.
In radiation therapy, a treatment for target tumor cells is administered by irradiating the cells with radiation. While X-rays are most commonly used in radiation-based treatments, the therapy that uses charged particle beams represented by particle beams highly dose-convergent upon targets (i.e., proton beams and carbon beams) is increasing in demand.
In radiation therapy, excess or deficiency of irradiation dose is likely to lead to side effects upon non-tumor normal tissues or to tumor recurrence. It is also demanded in particle therapy that a tumor region be irradiated with a dose that has been specified so that beam concentrates upon the tumor region as accurately as possible and as much as possible.
The use of scanning in particle therapy is increasing as a method of concentrating a dose. The scanning method is intended to bend a thin beam of particles via two sets of scanning magnets, guide the particle beam to any position within a plane, and thereby irradiate the inside of a tumor as if it were completely coated with the particles, to impart a higher dose to the tumor region only.
The scanning method has an advantage of there basically being no need of the patient-specific device, such as a collimator, that is used in a scattering irradiation method to form a distribution into the tumor shape. The scanning method also has an advantage in that various distributions can be formed easily.
To implement the scanning method, it becomes a vital step to draw up a plan with a radiation treatment planning system before starting actual irradiation. The radiation treatment planning system simulates the dose distribution within the patient's body by conducting numerical calculations based upon his/her in-vivo information obtained from CT images and the like. An operator, while referring to the calculation results supplied from the radiation treatment planning system, determines a particle beam irradiation direction, beam energy, an irradiating position, an irradiation dose, and other irradiation conditions.
The following briefly describes a general process employed for the simulation.
The operator first enters a target region to be irradiated with radiation. The target region is mainly entered onto slices of a CT image. The entered data is registered in the radiation treatment planning system by the operator and saved on a memory of this system as three-dimensional image data. If necessary, a position of a organ at risk whose exposure to the radiation is to be minimized is likewise entered and registered.
Next, the operator sets a prescription dose that is delivered in the target region, for each registered region on the slices. This setting operation is performed for the previously registered target region and organ at risk. For the target region, for example, a dose sufficient for necrosing the tumor is specified. Maximum and minimum values of the dose to be delivered to the target region are specified in most cases. For the organ at risk, on the other hand, a permissible dose is set as a maximum dose at which the organ is considered to be able to withstand the irradiation. If a plurality of target regions or organ at risks are present, relative importance of each can also be set as a weight.
Following the above, a determination is conducted of the irradiation conditions for achieving the dose distribution that satisfies the prescription dose. Until the dose distribution deemed appropriate has been obtained, the operator adjusts parameters relating to the irradiation conditions to be determined using the radiation treatment planning system. Widely adopted to set these parameters efficiently are methods using an objective function that represents a digitized deviation from the prescription dose, as described in JP-2002-263208-A, Non-Patent Document 1 (A Lomax, “Intensity modulation methods for proton radiotherapy”, Phys. Med. Biol., 44 (1999), 185-205) and Non-Patent Document 2 (Pedroni et al., “Experimental characterization and physical modeling of the dose distribution of scanned proton pencil beams”, Phys. Med. Biol., 50 (2005), 541-561), for example. The objective function is defined to become smaller the more the dose distribution fulfills the prescription dose. Calculations are repeated to search for the irradiation dose having the smallest value of the objective function, whereby an optimum irradiation dose is obtained.
Examples of a parameter determined by the objective function include an irradiation dose to various spots in particle beam scanning irradiation (this dose is hereinafter referred to as the spot irradiation dose). An example of a parameter search method using the objective function is the method of searching for the spot irradiation dose, proposed in Non-Patent Document 1. In this search method, doses to be imparted to calculation points within the target region or organ at risk are expressed in terms of dose matrix as the doses from the beams which have been delivered to each spot, and the objective function is calculated from the dose matrix with each search for the spot irradiation dose.
It is also known that as described in Non-Patent Document 2, scanning irradiation methods increase the beam size because of the influence of nuclear reactions in water or scattering inside the irradiation system.